1. Field of the Invention
The present invention relates to a cost effective Fischer-Tropsch process. More specifically, the present invention relates to a Fischer-Tropsch process in which water from a Fischer-Tropsch reactor is separated in a cost effective and energy efficient manner using a separation membrane.
2. Description of the Related Art
Prior approaches degas, de-oil, and/or distill Fischer-Tropsch reaction water to partially remove dissolved and entrained contaminants. However, the resulting reaction water still contains lower molecular weight hydrocarbons and oxygenates. As oxygenates are known to cause corrosion, while hydrocarbons may cause foaming, the resulting water product is of little or no commercial value and requires further treatment before recovery or preparation for disposal.
Thus, the water product is usually treated in expensive facilities in order to remove the contaminants. Typical treatment steps include alcohol stripping, anaerobic digestion, and biological oxidation. Such treatment steps serve to remove contaminants from the water product. Bio-treatment is costly, sensitive to operate, and generates solid wastes. Simple pH neutralization followed by offshore disposal requires regulatory variances, does not result in the recovery of any water, and requires large amounts of neutralization chemicals.
Methods for treating hydrocarbon synthesis wastewater are described in U.S. Pat. No. 3,966,633 and U.S. patent application 2002/0006969. Further, U.S. Pat. No. 6,225,358 discloses a method for producing heavier hydrocarbons from lighter hydrocarbons including converting synthesis gas into heavier hydrocarbons and removing contaminants from an aqueous byproduct stream. Contaminants are removed from the aqueous byproduct stream by concentrating the contaminants in a concentrator column and using the light hydrocarbons in a stripper column to remove the contaminants from the byproduct stream.
U.S. Pat. No. 6,533,945 provides a process wherein the wastewater of a hydrocarbon synthesis reactor, such as a Fischer-Tropsch reactor, is sent to a gasifier and subsequently reacted with steam and oxygen at high temperatures and pressures so as to produce synthesis gas.
The use of membranes is also well known and documented in the literature, including, by way of example, U.S. Pat. No. 5,525,143, which discloses a method and apparatus for the dehydration of gases utilizing hollow fiber membranes. An internal sweep of the permeate side of the membranes utilizes an aperture in the tubesheet at the product end of the module thereby sweeping the permeate side of the membrane with product gas. U.S. Pat. No. 5,942,119 discloses that the presence of hydrogen in the feed stream, permeate stream, or feed stream when present, either alone or in combination, causes a reduction in flux decay through molecular sieve membranes. U.S. Pat. No. 6,403,660 discloses a process for producing hydrocarbons involving allowing reactants forming part of a reaction medium in a reaction zone, to react at reaction conditions so as to form primary hydrocarbon products. By-product water, formed under the reaction conditions, is allowed to permeate through a porous membrane, thereby to be separated from the reaction medium. The separation is conducted in the reactor to separate the water from the catalyst. The water formed has an unfavorable or negative effect on the catalyst, e.g., a Fischer-Tropsch catalyst.
Ceramic membrane technology is well known and documented in the literature, including, by way of example, U.S. Pat. Nos. 4,781,831, 4,983,423, 5,009,781, 5,106,502, 5,108,601, 5,120,576, and 6,126,833.
“Applications of ceramic-membrane technology,” Fundamentals of Gas to Liquids, Petroleum Economist 2003, discloses the development of two technologies based on the applications of oxygen ion transport membranes. One of the technologies aims to combine the autothermal reformer and cryogenic air separation unit (ASU) into a single reactor and the other technology aims to replace the cryogenic technology in the ASU. The membranes are dense ceramic materials made from mixed metal oxides, and at high temperatures (over 700° C.), have both electronic and ionic conductivity, and are extremely selective and very fast in transporting oxygen.
Thermal oxidation, which converts harmful components to less polluting compounds, such as water vapor, carbon dioxide, and nitrogen oxides, offers the most widely proven solution to pollution from refinery, petrochemical, fine chemical, pharmaceutical, and other process industries. The use of thermal oxidizers to dispose of a wide variety of hazardous industrial wastes, especially tail gases containing a variety of sulfur compounds, is known. Descriptions of pollutant control processes can be found in Hazardous Waste Disposal by Thermal Oxidation, John Zink Company, 2001 and Thermal Oxidizers, Callidus Technologies, Inc.
It would be economically advantageous to be able to separate water from a Fischer-Tropsch overhead product without expensive condensation and vaporization equipment. It would be highly desirable to provide such a process while involving some of the useful technological advances to help simplify the overall process.